The invention relates to conveyor belts and, more particularly, to modular plastic conveyor belts constructed of rows of plastic belt modules hingedly interlinked by hinge pins.
Because they do not corrode and are easy to clean, plastic conveyor belts are used widely, especially to convey food products. Modular plastic conveyor belts are made up of molded plastic modular links, or belt modules, arranged in rows. Spaced apart link ends extending from each end of the modules include aligned apertures to accommodate a pivot rod. The link ends along one end of a row of modules are interleaved with the link ends of an adjacent row. A pivot rod, or hinge pin, journalled in the aligned apertures of the end-to-end-connected rows, connects adjacent rows together to form an endless conveyor belt capable of articulating about a drive sprocket.
In many industrial applications, conveyor belts are used to carry products along paths including curved, as well as straight, segments. Belts capable of flexing sidewise to follow curved paths are referred to as side-flexing, turn, or radius belts. As a radius belt negotiates a turn, the belt must fan out because the edge of the belt at the outside of the turn follows a longer path than the edge at the inside of the turn. To enable the belt to fan out, the apertures in the link ends on one end of each row are typically elongated in the direction of belt travel. The elongated apertures allow the belt to collapse at the inside of a turn and to spread at the outside.
The requirement of following a curved path causes problems not found in straight-running belts. For example, because the elongated apertures of conventional radius belts are identical in length across the width of the belt, only one or a very few of the link ends at the outside of a turn bear the entire belt pull. On a straight run, the belt pull is distributed across the entire width of the belt. Unless the outer link ends are specially bolstered, the belt pull strength rating is limited by the pull strength in a turn, which is often up to ten times less than on a straight. Thus, radius belts must be heavier and stronger than straight-running belts conveying the same load. Because the overall scale of structures and discontinuities on heavier belts is greater than on lighter belts, heavier belts are more likely to trip products such as beverage containers with small feet.
A conveyor belt having special edge modules with closer link end spacing and tapered pivot rod slots to improve the distribution of the pull at the outside of a turn is disclosed in U.S. Pat. No. 5,174,439, issued Dec. 29, 1992. A belt made up of those edge modules, however, still confines the belt pull in a turn to only a few closely spaced, thin link ends at the outside of the turn. The belt's strength in turns is less than on straight runs. This disparity in strength is greater the wider the belt. Thus, belt strength must be wasted to accommodate turns.
A sought-after feature in radius belts is a low turning ratio, i.e., the ratio of the radius of the tightest conveyor turn path to the width of the belt. Most radius belts have turning ratios of about 2:1 or greater. Thus, turns must be long and gradual, taking up otherwise usable space. Smaller turning ratios are generally limited by interference between the interleaved link ends as they collapse at the inside of a turn. A dual-pitch belt that collapses better at the inside of a turn is disclosed in U.S. Pat. No. 5,346,059, issued Sep. 13, 1994. The belt shown has shorter link ends on the inside half of the belt than on the outside half, which allows the inside edge to collapse tighter. The pivot rod apertures along each half, however, are slotted in transverse alignment with one another, and the load is borne by only the outermost and centermost link ends in a turn. On a straight run, belt pull is shared among many link ends. Consequently, the belt must be made much stronger or its load derated in order to handle the turns.